Didn't look all that out of the ordinary from the angle we got. Sure it's a bit extra for scrimmage, but again, the guy's a physical bottom pairing dman trying to make his way onto a team after failing in Calgary. You'd kind of expect him to be showing the coaches what he has to offer.
He likely didn't realize it was Macarthur nor did he know his concussion history. I have the utmost respect for Macarthur, but he clearly should not be playing if a hit like that puts him as out of it as he is. More concussions means you're more susceptible to them (I'd know, I have 4). His last one was apparently awful, so I can't imagine how people praise Gabriel Landeskog's article about them, talking about how you should take as much time as you need for every symptom to go away, and then not criticize Macarthur for coming back when he CLEARLY was not at 100%.
It was a stupid for training camp, but understandable given the player's circumstances, hit made by a bad player on a player who should not have been playing in the first place. The reaction was justified, but a hit like that does not concuss someone who isn't already still at least mildly concussed.